1. Field of the Invention
This invention relates generally to the field of vehicle propulsion systems. More particularly, the invention relates to an improved system and method for powering a vehicle using radio frequency signals.
2. Description of the Related Art
1. Introduction
Developing vehicles which reduce reliance upon fossil fuels is a matter of critical global importance. Fossil fuels (e.g., gasoline, diesel fuel, natural gas) are used in the vast majority of vehicles in the world because of fossil fuels' high energy density, fast refueling time, relatively low cost, and the maturity of internal combustion (“IC”) engines that run on fossil fuels.
The world's heavy reliance on fossil fuels for vehicular power has resulted in a number of problems and concerns. IC engine emissions, despite increasingly stringent controls in many countries, contribute significantly to air pollution and release significant quantities of carbon dioxide, potentially harming the ozone layer and/or contributing to global warming. Many of the world's largest reserves of fossil fuels are in politically unstable areas of the world. Moreover, the world has a finite supply of fossil fuel resources that can be practically obtained. While the exact remaining supply of fossil fuel resources is unknown and a matter of debate, there is universal agreement that at some point (perhaps 25 years, perhaps 100 years) supplies will peak and within an accelerated timeframe thereafter, supplies will be exhausted. Once the milestone event of peak supply and rapid depletion occurs, the cost of fossil fuel is likely to rise dramatically, further worsening the ongoing practicality for society to rely largely on fossil fuels.
A number of prior art systems have been developed and/or proposed over the last century that either eliminate the use of fossil fuels or reduce the use of fossil fuels. In particular, a number of prior art systems have been designed to utilize electricity sourced from the local power grid as energy for vehicle propulsion. Although a large percentage of electrical energy in the current US and world power grid is generated from fossil fuels, with electric-powered vehicles, countries have energy choices when generating electricity such as the type of fossil fuel used (e.g. natural gas, diesel, or coal), nuclear power, hydroelectric power, solar power, and/or wind power. However, with fossil fuel-powered vehicles the energy choices are limited to fossil fuels such as gasoline, diesel, and natural gas that are in liquid or gaseous form and are highly portable. Also, currently in the US, the cost of electric energy to produce a given watt output from an electric motor is typically less than the cost of gasoline to produce a given watt output from a gasoline engine. Of course, it is a complex analysis to compare an electric vehicle's overall efficiency to a gasoline vehicle's, but generally speaking, the energy costs for electric vehicles are less expensive per mile compared to similar gasoline vehicles.
Each prior art system developed and/or proposed in the last century to reduce fossil fuel dependence has its advantages and disadvantages, but to date, none has provided a solution which has the convenience and efficiency of a modern IC-powered automobile and offers a long-term solution that eliminates reliance on fossil fuels Several such prior art systems are reviewed here.
2. Current Vehicle Propulsion Systems
(a) Internal Combustion Engines
FIG. 1 illustrates a vehicle powered by a traditional internal combustion (“IC”) engine 116, the most common vehicle configuration today. Fuel from a fuel source 180 (e.g., an oil refinery) is transported to fuel pumps 181 maintained at gas stations. Users purchase the fuel at the gas station, and fill up a fuel tank 114 coupled to the vehicle. The fuel is then provided to the IC engine 116 via a fuel line 115.
The IC engine 116 burns the fuel and provides torque to a drive train 117 which interfaces with a transmission 182. The transmission is necessary to allow the engine to run at a rate (typically measured in RPM) which is not directly related to the speed of the vehicle. For example, when the vehicle is stopped (e.g., at a red light), the transmission allows the engine to keep running. Conversely, when the vehicle is moving at a high velocity (e.g., on the highway), the transmission allows the engine to run at a disproportionately low rate. A drive shaft 130 from the transmission 182 applies a force to cause the wheels 111 of the vehicle to rotate. FIG. 1 also illustrates a passenger compartment 100 for containing passengers 101 and a cargo compartment 102 (e.g., a trunk) for cargo 103.
The tires 111 of the vehicle illustrated in FIG. 1 are in contact with a standard road surface or track 150 which may be constructed using various materials (e.g., tar, concrete, steel, etc). In addition, various different materials may be used for the road or track bed 151 beneath the road or track surface 150 (e.g., gravel, wood, soil, etc). A certain amount of debris and/or precipitation 152 may also be found on top of the road surface in a typical outdoor environment.
(b) Electrically-Powered Vehicles
FIG. 2 illustrates a vehicle powered by an electric motor 124. A power source 190 (e.g., the US or international power grid) supplies power to a port 129 on the vehicle via an electric interface 191, which may include a set of connectors, a voltage regulator and/or a transformer. The port 129 is electrically coupled to a charger 127 which charges a set of batteries 122. The batteries provide power to the electric motor 124. A power split device 118 receives current from the electric motor and generates torque via a drive shaft 130 thereby causing the wheels 111 of the vehicle to rotate. In one embodiment, the power split device 118 may include a gearing transmission. Using the forward momentum of the vehicle, the power split device 118 powers a generator 120 which generates an electric current for charging the batteries 122 when the vehicle is breaking or going down hil by recovering energy from the forward momentum of the vehicle. Two examples of the electric vehicle shown in FIG. 2 are the Honda EV+ and the Saturn EV1.
(c) Hybrid Vehicles 
FIG. 3 illustrates an exemplary “hybrid” vehicle which runs on both gas and electricity. The power split device 118 in this vehicle allows couplings 125 and 117 to work together to power the drive shaft. More specifically, this vehicle includes both an IC engine 116 and electric motor 124 for generating torque on a drive shaft 130 via the power split device 118. As in the vehicle in FIG. 2, this vehicle also includes a generator 120 for charging the batteries 122 using the forward momentum of the vehicle (e.g., when the vehicle is breaking or going down hill). Examples of the vehicle shown in FIG. 3 include the Lexus® RX400h (note, however, that this vehicle is available with a second electric motor to drive the rear wheels) and the Toyota® Prius®.
FIG. 4 illustrates a hybrid vehicle which includes both an IC engine 116 and electric motor 124. However, unlike the vehicle shown in FIG. 3, this vehicle includes an electricity port 129 and charger 127 for charging the batteries using power from an electric power source 190 (e.g., the US power grid) through electricity interface 191. Given the fact that this vehicle can charge using a standard electrical connection, the batteries 122 of this vehicle are typically larger and can supply more power than the batteries of the vehicle shown in FIG. 3.
(d) Hydrogen Vehicles
FIG. 5 illustrates a vehicle which is similar to the vehicle shown in FIG. 2 but which uses a hydrogen fuel cell 196 to charge the set of batteries 122. The batteries 122 are used in hydrogen-powered vehicles because the fuel cell 196 cannot produce sufficient instantaneous power levels for acceptable vehicle acceleration. Like gasoline, hydrogen from supply 198 is provided to the vehicle via a fuel port 112 and is stored within a hydrogen storage chamber 194.
One problem with hydrogen vehicles is that hydrogen is not readily available as a fuel source. Generally, fossil fuels (e.g. natural gas) are used to produce hydrogen, but this defeats the purpose of a non-fossil fueled vehicle. Although hydrogen may also be produced using electrolysis powered by a electrical source, this process is inefficient and makes hydrogen an unreasonably expensive fuel source.
(e) Conductively-Powered Vehicles
FIG. 6 illustrates a prior art conductively-powered electric vehicle in which power rails/cables 691 are coupled to a power source 190. A power cable 692 is used to establish an electrical connection between the power rails/cables 691 and an electrical port 129 on the vehicle. The port 129 is electrically coupled to a charger 127 which powers a set of batteries 122 which, in turn, provide power to an electric motor 124. The electric motor 124 creates torque to power a drive shaft 130 which rotates a set of tires or wheels 111. As illustrated in FIG. 6, the power source 190 may be connected to the train by a combination of power rails or cables 691 and track 150 (e.g., as in the case of electrically powered trains).
There are many examples of prior art electric vehicles that are powered conductively by an external power source that is physically (i.e., conductively) attached. Such vehicles follow the general architecture illustrated in FIG. 6. Power source 190 is coupled through power connection 693 to power rail(s) or cable(s) 691. Unlike electricity interface 191 of FIG. 2, which couples electricity from a recharging station at a fixed location, power rail(s) or cables(s) 691 in FIG. 6 couple electricity with long and continuous electrical conductors for carrying power over the extent of the vehicle's intended travel (e.g., the two overhead power cables over electric bus routes in San Francisco), and the power cable 192 of FIG. 3 is replaced by power cable 692 that has a conductive interface 694 that rolls or slides on power rail(s) or cable(s) 691 (e.g., the two overhead connectors on electric buses in San Francisco that couple to the two overhead power cables). In some cases, the two conductors of power source 190 are split between a potentially dangerous non-ground single conductor power connection 693 coupled to power rail or cable 691 (e.g. the so-called “third rail” of the New York City subway system), and a harmless ground connection 693 coupled to a conductive track 150 (e.g., the track rails of the New York City subway system). In such a system the power rail or cable 691 typically is physically inaccessible to prevent accidental contact by a person or animal that might result in electric shock.
The charger 127 and batteries 122 shown in FIG. 6 provide temporary power in the event of intermittent connection loss to the power source. However, such vehicles may also be constructed without battery backup with a direct power connection from electricity port 129 to electric motor 124. Also, such vehicles often have a direct mechanical coupling 130 from the electric motor 124 and the tire or wheel 111.
Electric trains or trolleys with powered overhead wires are a common example of conductively-powered electric vehicles. Toy slot cars are another example, with two wires embedded in the track that are coupled to two-wire mesh connectors on the bottom of the car.
Less well known is DICK FRADELLA, ELECTRIC HIGHWAY VEHICLES . . . TECHNOLOGY ASSESSMENT OF FUTURE INTERCITY TRANSPORTATION SYSTEMS published in 1976 titled by University of California at Berkeley's Institute for Transportation Engineering. In this publication, a conductive rolling contact system for electric vehicles on highways was proposed. Using this system, an electric vehicle would have an extended tether that would connect to recessed power strips on the highway and conductively draw power from the highway. According to a website apparently maintained by the paper's author (http:llhome.earthlink.net/˜fradella/car.htm) the US DOT and DOE rejected such a conductive system out of concerns that people might be electrocuted by the conductive power strips. Independent of electrocution risk, another substantive issue was that cars so connected to the conductive power strips would be quite restricted in their maneuverability, much like toy slot cars, in order to utilize the conductive power source. This would require substantial changes to existing car designs and driving procedures. In addition, debris or precipitation 152 (e.g. snow, ice, mud, oil, gravel, trash) could obstruct or interfere with the conductive interface.
(f) Inductively-Powered Vehicles
Another type of vehicle that does not directly rely upon fossil fuels is an inductively-powered electric vehicle. Electromagnetic induction is formally defined as the production of electrical potential difference (voltage) across a conductor located within a changing magnetic flux. A practical example of induction is seen in a transformer. There is no conductive connection between the primary and secondary sides of a transformer; the primary and secondary sides are simply coils of wire in close proximity to one another. When alternating current (AC) is applied to the primary side of a transformer, it induces an AC current in the secondary side of the transformer.
Induction can also be achieved between two parallel non-connected conductors in close proximity to one another. FIG. 7 illustrates how this principle has been applied in prior art inductively-powered electric vehicles. Power source 190 powers primary power supply 791, providing AC power (e.g. 10 to 25 KHz AC is used in a system designed by Wampfler AG of Rheinstrasse, Germany; see www.wampfler.com), which is coupled through power connection 792 to primary cable 794 (which loops back through another cable to power connection 792). Typically, the primary cable 794 is buried within a few centimeters of the surface of the road or track bed 151 or above the surface in an insulated enclosure.
Secondary pickup 795 contains a long conductive element which, depending on the clearance from the vehicle to the road or track bed, may have to be extended from the vehicle body. Secondary pickup 795 must be quite close (within a few centimeters) and maintained in parallel to the primary cable 795 as the vehicle moves. Secondary pickup 795 is coupled to secondary regulator 793 which serves to regulate voltage fluctuations caused by variations in the distance and alignment between secondary pickup 795 and primary cable 794. Secondary regulator 793 is coupled to charger 127, which is coupled through cable 126 to charge batteries 122 which couple through cable 123 to electric motor 124, and/or secondary regulator 793 may be coupled directly to electric motor 124, if the vehicle is to operate only from inductive power without batteries. Electric motor 124 is coupled by mechanical coupling 130 to drive tire or wheel 111.
There are only a few examples of inductively-powered electric vehicles. Wampfler AG, for example, has deployed several inductively-powered systems for electric vehicles. Inductive power has been used for vehicles on manufacturing floors and in other controlled environments. Because of the precise and close relative spacing requirements (e.g. ±25 mm in one Wampfler AG system) for inductive primaries and secondaries, inductive power transmission is difficult to apply generally to vehicles which may have to operate in a hostile outdoor environment. For example, the debris or precipitation 152 shown in FIG. 7 may obstruct a secondary pickup 795 with only a few centimeters of position tolerance.
(g) Wireless Power Transmission
Transmission of power through radio waves (“wireless power” or “RF power”) was pioneered by Nikola Tesla. His Tesla Coil, which demonstrates the principle of RF power transmission, has been a common fixture in science museums and science classes for decades, but it has found little practical application because it is quite inefficient as a wireless power transmitter. Nonetheless, Tesla envisioned a world where wireless power was ubiquitous. There was even one report (see New York Daily News, Apr. 2, 1934, “Tesla's Wireless Power Dream Nears Reality”) that Tesla was developing a wirelessly-powered car, details of which were “closely guarded secrets.” However, the speculations of this article were never corroborated. A World-Wide Web resource site on Nikola Tesla (www.tfcbooks.com) has compiled a significant amount of information related to Tesla's work. Regarding electric-powered automobiles, the web site states: “While there is no corroborated evidence that Tesla ever built an electric automobile, he is known to have encouraged others to pursue the idea of electric propulsion.” (http://www.tfcbooks.com/teslafaq/q&a—015.htm).
Since Tesla's first work with wireless power transmission one hundred years ago, there have been a number of other wireless power transmission experiments and demonstrations.
In the 1960s, William C. Brown helped develop the rectifying antenna (“rectenna”), which converts radio waves to direct current (“DC”). An exemplary rectifying antenna is illustrated in FIG. 8. As taught by William C. Brown and others, a rectenna, when exposed to radio waves (typically in the microwave band) receives transmitted power and converts the microwave power to DC power. A typical rectenna consists of multiple rows of dipole antennas where multiple dipoles belong to each row. Each row is connected to a rectifying circuit which consists of low pass filters 801 and a rectifier 802. A rectifier is typically a GaAs Schottky barrier diode that is impedance-matched to the dipoles by a low pass filter 801. A low-pass filter 801 is a device that cuts frequencies off above a certain point and allows all other frequencies to pass through. Rectennas may also employ capacitors 803 to store charge as it flows through the receiving sub-systems.
Rectenna technology is well-understood by those skilled in the art, and there have been a number of prior art refinements, including U.S. Pat. No. 3,887,925 and U.S. Pat. No. 4,943,811. Efficiencies as high as 90% have been achieved from transmission of power to reception of power by a rectenna.
Transmission of wireless power to rectennas has been used for, and has been proposed for, many applications. In 1964, William C. Brown demonstrated the transmission of wireless power to a tethered model helicopter with a rectenna.
In the 1980s, the SHARP (Stationary High Altitude Relay Plafform) project resulted in a microwave-powered electric airplane with a 15-foot wing span. Its maiden voyage was in 1987, and it eventually was flown to a height of 1500 feet by beaming 12 kilowatts of RF power from the ground to the plane for more than an hour. Only a small fraction of the transmitted RF was received by an on-board rectenna. The work is currently described at http://friendsofcrc.ca/SHARP/sharp.html and in U.S. Pat. Nos. 4,943,811; 5,045,862; 5,321,414; 5,563,614 and Canadian Patent Nos. 1,307,842; 1,309,769; 2,006,481; 2,011,298. A ground power transmission system transmitted microwaves to the plane.
Hobbyists and students have also used wireless power for powering motors for other applications. Akshay Mohan describes experiments he conducted in 2002 with wireless power transmission. His initial goal was to develop a vehicle that could divide itself so that at one point in time it could be a family car, and then divide itself at another point in time so that each part individually could be a vehicle a person could drive. He thought initially about coupling independent suspension and transmission mechanisms, and then considered using wireless power transmission to distribute power amongst the various independent parts of the vehicle. The power transmitted was very low power, and was used to power a motor removed from a toy car. The experiment is described at the following URL: http://www.media.mit.edu/physics/pedagogy/fab/fab—2002/personal_pages/akshay/mit.edu/index42.html